Brazing is a process of joining two metals by heating them to a suitable temperature and by using a filler metal which has a melting point lower than that of the metals being joined. The filler metal flows into the joint gap by capillary action and upon solidification forms a strong leak-tight joint.
Aluminum and some of its alloys are generally brazed by using aluminum-silicon based brazing compositions designated by the American Welding Society (AWS) as BAlSi filler metals. The lowest melting composition of this family is the BAlSi-9 composition which has a liquidus temperature of 582.degree. C. (1080.degree. F.) (See: Specification for Filler Metals for Brazing, ANSI/AWS A518-89).
Generally, when aluminum or aluminum alloys are brazed, their solidus temperature must be higher than the liquidus temperature of the filler metal to prevent distortion or warpage of the base metal due to overheating. For this reason, the BAlSi filler metals are only suitable for brazing pure aluminum and some of its alloys, e.g. 3003, 3004, 1100, 5005, 5050, 6063 as shown in FIG. 1. However, there are many other aluminum alloys used in the industry which need to be permanently joined such as by brazing. These alloys include 6061, Cast 443, Cast 356 and some of the recent aluminum-lithium based alloys such as Weldalite 049.TM.. At present, a suitable filler metal is not available for proper brazing of these alloys. There remains a need in the art, therefore, for an aluminum brazing alloy for joining these low melting aluminum alloys.